Atrial fibrillation (AF) is an arrhythmic condition of the heart in which the normal cardiac electrical impulses spread through the atrium in an incoherent manner, preventing the atrium from efficiently delivering blood to the ventricle. It is estimated that over 2.2 million Americans and 4.5 million EU citizens suffer from atrial fibrillation. Annual costs in the U.S. related to AF are approximately $16 billion. There are approximately 300,000 new AF cases each year. Contributors to AF incidence include the aging population, and many conditions including hypertension, cardiomyopathy, structural heart disease, diabetes, sleep apnea and obesity. Approximately 15% of stroke cases are due to clots originating in blood pooling in the atria due to AF. AF is classified into several types, primarily paroxysmal, and chronic, which includes persistent, and permanent with subtypes, depending on the presentation and associated morbidities. First-line treatment of AF is pharmaceutical, either through rate control, rhythm control, anticoagulation, or some combination. Due to the multiple types of AF, and the many agents and protocols used, the overall success of drug treatment cannot be accurately stated; some estimates are that overall drug efficacy is <40%. Additionally, some drug treatments have side effects that reduce quality of life or present risks.
When drug treatment is unsatisfactory, AF can be treated by destruction of the paths through which the erratic electrical impulses are spread. The destruction can be accomplished from either the epicardial or the endocardial surface, and by either mechanical means, such as the Cox Maze surgery in which tissue dissection disrupts those unwanted electrical pathways, or by application of energy to the tissue. Energetic ablation can be performed using radio frequency (RF) energy, microwave energy, ultrasonics, or cryotherapy, among others. The goal of ablation is to create a continuous, fully transmural line of necrosed tissue, which are able to conduct electrical signals across the line, effectively creating an electrical fence. However, today's ablation techniques are complex and have not reached high efficacy, thereby limiting their clinical utility. A particular problem is that clinicians cannot easily determine during the procedure whether the ablation produced is likely to interrupt conduction permanently.
About 20% of ablations are epicardial; this route is chosen when other treatments have failed, and when cardiac surgical procedures are also needed, as epicardial ablation generally requires heart bypass. The remaining 80% are endocardial, performed using a percutaneous catheter inserted into a vein, then into the right atrium, and then via a trans-septal puncture through the septum into the left atrium. The most common ablation techniques attempt to create circumferential ablations around the ostia, the locations where the pulmonary veins (PV) enter the left atrium. This isolates the disorganized signals arising in the veins from the atrium, without inducing stenosis due to pulmonary vein ablations. However, of the approximately 1 million AF patients in the US not successfully treated with drugs, only about 100,000 are treated by ablation annually. More are not treated by ablation due to its difficulty and the wide variation in treatment efficacy. Approximately 40% are repeat ablation procedures.
Endocardial catheter ablation is currently a two to six hour procedure performed by electrophysiologists (EPs). Much of this time is needed for a spot by spot creation of the required circumferential ablations using the ablation tools currently available, along with the time spent to verify conduction block, and follow-up during the procedure to insure that conduction block has been maintained, and when not, reablate specific locations as determined via conduction measurements. Reported long term success rates range from 20-70%. The efficacy decreases as AF progresses. To achieve even these results, approximately 40% of patients require repeat procedures at significant cost to the healthcare system, along with the radiation exposure from imaging and other risks to the patient and to the clinician inherent in these procedures. Market research indicates that both the variations in efficacy and the lengthy duration of the procedure are primarily due to uncertainty on the part of the clinician during the procedure as to whether the ablation lesion is continuous, complete, permanent, and transmural.
Present commercial minimally invasive catheter ablators consist of numerous single point ablation catheters, as well as a number of more recent devices, including a balloon catheter utilizing a laser energy source, a balloon catheter utilizing cryothermal energy, a multi-electrode ablator, utilizing RF energy and various robotic systems to maneuver catheters through the vascular system into the heart.
The two balloon ablators are applied in a similar manner, as they are inserted via a catheter and placed at the ostium, or intersection of the pulmonary veins with the atrial wall. Their placement limits their application to only electrically isolating unwanted signals around the pulmonary veins from the rest of the heart. Electrophysiologists, who perform the ablation procedures have also indicated that follow-up spots still need to be ablated, and there have been reports of injuries to surrounding tissue such as the phrenic and vagus nerves, and stenosis of the veins. Since they occlude blood flow through the vein, the balloons need to be adequately stiff to oppose the pressure from the blood flow. This is desirable in order to maintain their position and contact with the target tissue during the ablation cycle, otherwise they are less likely to achieve a continuous abaltion.
The multi-electrode array ablator, mounted on a Nitinol frame, can be used to map, ablate, and verify the ablation line by measuring conduction block, across the ablation line, around the pulmonary veins or in other target areas. Although this device can use its Nitinol frame to more readily conform to the target surface, while using a low level of applied force, which can provide enhanced contact to maximize ablation energy transfer, electrophysiologists have reported that this requires additional discrete point ablations to be performed. This increases procedure time and reduces the likelihood of generating a continuous, fully transmural ablation. To maintain contact between the array and target tissue requires the electrophysiologist to continue to apply force during the ablation cycle, similar to point and balloon ablators. Because each ablator electrode in the array resides in a continuous ring, it may not satisfactorily conform to the target tissue's topography.
There are a number of robotic systems in development and already commercialized that augment the clinician's ability to maneuver the catheter to the selected target in the vascular system, including chambers in the heart. One such robot system allows a magnet to direct a catheter to a target and hold it against the target. It is designed to maneuver and hold point ablators. Point ablators take significant procedure time and do not necessarily generate continuous lesion lines to block unwanted electrical pathways. In addition the robots are very expensive.
However, present methods and technology do not provide features for locating and fixing in place the ablative element(s) that are physically separate from the mechanism for performing the ablation. This lack of separation limits the capability of devices based on these prior inventions to accurately locate the tissue volume to be ablated with respect to the pulmonary vein target at a location which minimizes the possibility of pulmonary stenosis, while also adjusting the contact of the ablative element(s) to provide intimate and accurate contact of the ablative element(s) with the atrial tissue and thereby form an ablated volume that fully encloses the ostium of the pulmonary vein.
In addition, the balloons and multi-electrode array are constructed and arranged to apply a continuous ablation line. These technologies are limited because they must be in continuous contact throughout the ablator-tissue contact range. They have problems maintaining that contact during the ablation cycle.
Also, prior systems employ primarily only point ablators to generate lesion lines beyond the pulmonary vein isolation technique, which creates a circumferential ablation around the pulmonary veins. However, this is a difficult procedure, which requires a high level of skill, exposes the clinicians and patient to radiation during imaging, extends procedure durations and reduces efficacy.